1. Field of the Invention
This invention relates to hotmelt adhesives for bonding DVDs which have a sandwich-like structure and to a process for the production of such DVDs.
2. Discussion of the Related Art
DVD is the abbreviation for digital versatile discs or digital video discs. These are optical storage media similar to the known compact disc (CD). The main difference between DVDs and CDs is the considerably higher density of musical information, graphic information or data stored on DVDs. This higher data or information density of the storage medium imposes greater demands on the production process and the materials used therein.
One possible construction of a DVD is schematized in FIG. 1 which shows a so-called "DVD 5". This DVD consists essentially of two halves, it is one-sided, carries one layer of information and has a storage capacity of 4.7 gigabytes. In FIG. 1, the information-carrying layer is denoted by the reference 1 while the information-free top layer is denoted by the reference 2. The laser beam needed to scan the information is denoted by the reference 7.
Accordingly, the structure of a DVD differs from that of the standard CD (compact disc) in that the DVD has a sandwich construction. Whereas CDs consist essentially of a 1.2 mm thick disc of polycarbonate or (poly)methacrylate resin, DVDs are made from two 0.6 mm thick discs. Today, polycarbonate is almost exclusively used for these discs. A refined data structure coupled with a laser of short wavelength enables one information layer of a DVD to carry around 4.7 gigabytes of information compared with the 680 megabytes of information carried by a standard CD.
The sandwich construction of DVDs means that the two layers 1 and 2 have to be joined together. Originally, solvent-based adhesives were used to bond the two layers. More recently, hotmelts, UV-curable solventless liquid adhesives and UV-crosslinkable hotmelt adhesives have been proposed.
The production of a DVD essentially involves the following crucial steps:
The polycarbonate or poly(meth)acrylate blanks have to be produced by injection molding. PA1 The blank 1, which carries the layer of information, is coated with a reflective layer--generally a metal layer, for example of aluminium, applied by vapor deposition. PA1 This reflective layer has to be protected against corrosion immediately after production. In one known embodiment, therefore, a lacquer film cured by UV radiation is applied to the reflective layer. An alternative method is to apply a protective film of a thermoplastic material. PA1 The information-free blank 2 can be printed with graphics and text by various methods. PA1 The blanks 1 and 2 are bonded together with an adhesive. PA1 reliable bonding of the two substrate halves PA1 corrosion control for the reflective (metal) layer so that there is no need to apply a separate anti-corrosion layer to the reflective layer PA1 the pigmented hotmelt adhesive may additionally serve as a covering and contrast-increasing background for a printed text/graphics layer applied as a mirror image to the substrate layer 2 so that there is no need to apply another covering background graphic.
DE-A-32 24 647 describes a process for the production of optical video discs and digital audio discs. In this process, the information-carrying layer, after coating with a reflective film of aluminium, is provided with a protective film of a hot-melting film-forming material which melts at a predetermined temperature and hardens at room temperature. In one embodiment, the hot-melting film-forming material is also the adhesive for joining the two disc substrate halves. In another embodiment, the film-forming material is coated with another adhesive which is tacky at room temperature. The two disc substrates are joined together with this adhesive. DE-A-32 24 647 does not mention anything about the composition of the hot-melting film-forming material or the adhesive tacky at room temperature. In addition, there is nothing in the teaching of DE-A-32 24 647 to indicate whether these adhesive materials are also suitable for the production of DVDs.
DE-A-32 46 857 describes an optical disc comprising a pair of substrates of which at least one has information "pits" formed on one surface. A metallic layer of a reflective film is applied to the layer of information pits while a protective resin layer is applied to the metallic reflective layer. The protective resin layer carries a layer of adhesive by which the two substrate halves are joined together. According to DE-A-32 46 857, the adhesive is selected from hotmelt adhesives containing a mixture of one or more thermoplastic elastomers as the basic polymer. In addition, it is clear from the teaching of DE-A-32 46 857 that the hotmelt adhesives described therein contain standard tackifying resins and additives, for example fillers, antioxidants or UV absorbers, to increase their resistance to heat and weathering. It is also stated that the melt viscosity of the adhesive should not exceed a value of 1,000 poises at 160.degree. C. so that the adhesive does not have any adverse effects on the vapor-coated metallic film. Nothing is said about the suitability of these adhesives for the production of DVDs.
According to the teaching of DE-A-38 40 391, the use of hotmelt adhesives in the bonding of substrate pairs for the production of video discs and digital audio discs is problematical on account of their poor thermal stability. According to this teaching, the use of UV-curable monomer compositions as adhesive can cause corrosion of the substrate. DE-A-38 40 391 proposes the use of UV-cosslinkable hotmelt adhesives for bonding the substrate pairs in the production of video discs. Nothing is said about the suitability of these UV-crosslinkable hotmelt adhesives for the production of DVDs.
As mentioned at the beginning, DVDs are distinguished from CDs and optical discs (also known as laser discs) by a higher data or information density of the storage medium so that they impose more stringent demands on the production process and the materials used therein, for example adhesives. UV-curing adhesives tend to undergo adhesion failures, for example under impact, because they are generally very brittle after curing. Similarly, most UV-curing adhesives and other reaction adhesives undergo relatively high natural shrinkage. The resulting force applied by the adhesive to the reflection layer can have an adverse effect on the information content and the operational reliability of the DVD.